The invention relates to a method and device for producing permanently magnetized objects, and multipolar rotors of small dimensions in particular.
The method according to the invention relates to the production of a magnetic object to be molded in a molding device from a mixture of grains of magnetic material and hardening binding agent, said object having pole regions of small dimensions, the mixture being subjected in a molding cavity of a molding body of the molding device to temperature changes, gravity, mechanical forces or magnetic forces, or combinations of these forces.
It is generally known how to produce magnetic bodies by means of magnetic material bound by resin or a suitable synthetic material according to which method pulverized or granulated sintered magnetic material such as SmCo.sub.5, and Sm.sub.2 Co.sub.17 is processed, while adding a suitable binding agent, into semi-finished product magnetic elements in a mass production process. By using the correct grain size for the magnetic material, desired filling factors can be obtained. The produced semi-finished magnetic elements, having no or only slight net-magnetization, can then be processed further, e.g., to adhering strips, in which process they finally can be magnetized permanently so as to perform the function they have been designed for.
The art does not teach how the above-mentioned magnetic material can be used in a production process in such a way, that in subsequent process steps multipolar permanently magnetized objects can be produced which incorporate the desired magnetic properties and magnetic pole configurations. In the case of pole regions of small dimensions, to be realized, e.g., on a multipolar rotor for a stepper motor with a diameter of up to 4 mm and sixty poles with alternatingly N- and Z- poles along its periphery, it is desirable to provide the product that is to be manufactured with strong magnetic poles already in the production stage. Although it is possible with the known method to establish high filling factors with the grains, it is extremely difficult, not to say impossible, to magnetize those afterwards to poles with a width of about 0.2 mm.
In order to solve this problem, the method according to the invention is characterized by the reduction of a strong, permanent magnet to fragments of fully magnetized anisotropic permanent magnet material, the reduction of the fragments of fully magnetized anisotropic material to grains, until all grains are smaller than the width of a pole region, mixing those grains with the hardening binding agent, inserting the mixture into the molding device, and ensuring that the mixture hardens in the molding device, thus providing the permanently magnetized object as the final product.
This method can be used particularly to obtain grains that are smaller than 150 .mu.m or of another required size from desired, fully magnetized anisotropic permanent magnet material.
The invention also provides a method and a device for reducing fragments of fully magnetized anisotropic permanent magnet material, in which the fragments are introduced between grinder bodies of which at least the surfaces that face the fragments are made of the same magnetic material. Specifically the fragments are inserted between two grinder bodies of which at least the surfaces that face each other have mutually opposite magnetic poles.
Moreover, the invention provides a method in which the mixture inserted in the molding device is brought from at least a second molding body that is larger than but identical in structure to the first molding body to the first molding body through a passage member, with the periodical replacement of said filled first molding body by an identical molding body that is to be filled next, the first filled molding body providing a permanently magnetized object as the final product.
A special characteristic of this method and molding device is that the mixture is brought to the first molding body, while being subjected for at least a part of the passage member to magnetic forces originating from magnetic means near the surface of the passage member's inner circumference.
Another feature of the molding device is that at least the inner circumference of a cross-section of the passage member is identical in shape with the inner circumference of a molding body, the dimensions of the inner circumference gradually declining from that of the second molding body to that of the first molding body. The embodiment in which each cross-section near the inner circumference of the passage member is similar in structure to that of the molding body and the inner circumference of the passage member extends conically from the second molding body to the first molding body is preferred.
Additionally the molding device is characterized by an axially symmetrical presser means that, under axial displacement thereof in the molding device, presses the mixture in the direction of the first molding body, while the presser means, which is composed of a mandrel protruding at least partially into the molding device and having a closely fitting sleeve between the molding device and the mandrel for pressing the mixture to the first molding body, is preferred.
Another feature of the molding device is a mandrel provided with magnetic poles, on at least a part of its surface. The magnetic poles are aligned with poles that are situated near the inner circumference of the second molding body and of at least part of the passage member.
Such a molding device may comprise fixed ribs that extend partially or entirely between interpolar areas between the poles of the magnetic means in the mandrel and of the magnetic means in the molding device, the sleeve comprising a periphery which closely fits to these ribs and which can be displaced up to the passage member.
A system according to the invention is characterized by the above-mentioned molding device and grinding device, to which suitable supply and discharge means have been added.